The present invention relates to nuclear magnetic resonance (NMR) spectroscopy and, more particularly, to novel methods for 1-, 2-, and 3-dimensional metabolite imaging and quantification using NMR spectroscopy.
It is highly desirable to utilize spatially-localized in-vivo nuclear magnetic resonance (NMR) spectroscopy to acquire information about the distribution of certain low-abundance or low-sensitivity elements, such as .sup.31 P and the like. By providing direct and non-invasive access to information about the concentration of relatively high-energy phosphate metabolites and other phosphorus compounds in living cells in normal and disease-comprised tissues, the state of the medical diagnostic arts can be advanced. Current techniques for localizing the desired NMR spectroscopy signals in human and intact animals has either invariably utilized small surface NMR detection systems, or been restricted to data acquisition from only a single selected volume element (voxel) at any one time. Thus, the acquisition of in-vivo spectroscopic images, where the term "image" is utilized to signify a display of the spatial variation of one or more physical properties, such as chemical shift spectra, which images are spatially resolved in 1, 2 or 3 dimensions (1D, 2D, 3D) has for many years proved impractical with NMR techniques monitoring free-induction-decay (FID) response signals and employing 2D, 3D and 4D Fourier transformation (FT), such as the techniques proposed by Brown et al. in U.S. Pat. No. 4,310,190 (1982). It will be understood that since each chemical shift spectrum itself has two dimensions (signal intensity and chemical shift, in parts per million or ppm.), the images involved in the present invention all have one more dimension d' than is usually displayed. While it is known to acquire spectroscopic images of certain elements, such as .sup.31 P, by the utilization of 2D, 3D and 4D FT techniques, employing NMR spin echoes, such as described in U.S. Pat. Nos. 4,506,223 and 4,567,440, these techniques are unsuitable for observing spatial distribution of important .sup.31 P metabolites that have T.sub.2 values so short as to decay substantially away during the spin-echo time interval. To date, these techniques have not allowed images to be obtained of short-T.sub.2 metabolites, such as adenosine triphosphate (ATP), phosphodiesters (PD), and the like. Additionally, even those .sup.31 P metabolites which have been observed, such as phosphocreatine (PCr), inorganic phosphate (Pi) and phosphomonoesters (PM), are observed as spectral signals attenuated by the different T.sub.2 decays and may not provide an accurate measure of the relative concentration of these metabolites. Additionally, reported scan times of up to four hours for 2D localized .sup.31 P spectroscopy, using a spin-echo 3DFT method, are too long for reasonable human patient studies and, in fact, no actual patient studies utilizing these techniques appear to have been reported. It is therefore highly desirable to provide a method of, and apparatus for, obtaining a NMR spectroscopic metabolic image with full-body volume, multiple-voxel localization, especially to voxels of size from about 4 cm.sup.3 to about 40 cm.sup.3, of substantially undistorted FID spectral data. It is also highly desirable, since metabolite resonances are derived directly from freeinduction decays with minimal timing delays, that the integrated FID signals be so obtained as to more accurately represent the relative concentrations and absolute values of the metabolites present.